Why there is a need for bonds for atoms?
It is a universal rule that everything in this world tends to be more stable. Atoms accomplish stability by attaining electronic configuration of noble gases (He, Ne or Ar, etc) i.e. ns2 np6. Having 2 or 8 electrons in the valence shell is an indication of stability. Obtaining 2 electrons in the valence shell is called duplet rule while obtaining eight electrons in the valence shell is called octet rule.
The noble gases do have 2 or 8 electrons in their valence shells. It means all the worthy gases have their valence shells entirely filled. Their atoms do not have vacant space in their valence shell to accommodate additional electrons. Therefore, noble gases do not gain, lose, or share electrons.
That is why they are non-reactive. The value of the noble gas electronic configuration depends on the fact that all other atoms try their best to have the noble gas electronic configuration. For this cause, atoms combine with one another, which is called chemical bonding. To put it simply, atoms form chemical bonds to accomplish stability by acquiring inert gas electronic configuration. An atom can accommodate 8 electrons in its valence shell in 3 methods:
- By offering valence shell electrons (if they are less than 3) to other atoms.
- By acquiring electrons from other atoms (if the valence shell has five or more electrons in it).
- By sharing valence electrons with other atoms.
So, to attain stability atoms form chemical bonds.
Chemical Bond Definition
A chemical bond is defined as a force of attraction between atoms that holds them together in a compound. In other words, during bond formation, there is some force that holds the atoms together.
Types of Chemical Bond
The valence electrons, which are associated with chemical bonding, are described as bonding electrons. They normally reside in the incomplete or partially filled outermost shell of an atom. Depending upon the method of how these valence electrons are involved in bonding, they result in the following four kinds of chemical bonds:
- Ionic Bond
- Covalent Bond
- Dative Covalent or Coordinate Covalent Bond
- Metallic Bond
The elements of Group-1 and Group-2 being metals have the tendency to lose their valence electrons forming positively charged ions. Whereas non-metals of Group- 15 to Group-17 have the tendency to get or accept electrons. They are electronegative elements with high electron affinities.
If atoms coming from these 2 different groups, metals and non-metals, are enabled to react, a chemical bond is formed. This type of chemical bond, which is formed due to the complete transfer of an electron from one atom to another atom, is called ionic bond.
The development of NaCl is a fine example of this kind of bond.
Salt is a simple compound formed by Sodium (Z =11) and chlorine (Z= 17) atoms. The ground state electronic configuration of these components is shown listed below:
The frames show electrons in the valence shells of these elements; sodium has only one electron and chlorine have 7 electrons. Sodium being an electropositive element has the propensity to lose electron and chlorine being an electronegative element has the tendency to acquire electron. Therefore, they form positive and negative ions by losing and acquiring electrons, respectively. They attain the electronic configuration of the nearby noble gases.
By losing one electron from the outer shell, sodium becomes Na+ ion and it is entrusted to 8 electrons in the 2nd shell which will now become the valence shell. By acquiring one electron, the chlorine atom now also has eight electrons in its outermost shell and ends up being CI– ion. Both of these atoms are now changed into oppositely charged ions. They support themselves by integrating with each other due to the electrostatic force of attraction between them such as:
It is to be kept in mind that only valence shell electrons take part in this kind of bonding, while other electrons are not included. In such kind of reaction, heat is usually given out. The compounds formed due to this kind of bonding are called ionic compounds.
The metallic bond is defined as a bond formed in between metal atoms (positively charged ions) due to mobile or free electrons.
The different properties shown by metals such as high melting and boiling points, excellent conductions of heat and electrical energy, tough and heavy nature, suggest the existence of various kinds of chemical bonds between atoms of metals.
In the case of metals, the hold of the nucleus over the outermost electrons is weak because of large-sized atoms and a greater number of shells in between the nucleus and valence electrons. Moreover, because of low ionization capacities, metals have the propensity to lose their outermost electrons quickly. Resultantly, these loose or free electrons of all metal atoms move easily in the spaces between atoms of metal.
None of these electrons is connected to any particular atom. Either they come from a typical pool or belong to all the atoms of that metal. Nuclei of metal atoms appear submerged in sea of these totally free mobile electrons. These mobile electrons are accountable for holding the atoms of metals together forming a metal bond.